Mastocytosis encompasses a spectrum of uncommon disorders characterized by proliferations of mast cells; bone marrow derived cells that reside in connective tissue and play an important role in anaphylaxis and allergy. Mast cell granules contain histamine, tryptase and other mediators that is largely responsible for the clinical manifestations of the disease.

Mastocytosis may be localized to the skin, as occurs in urticaria pigmentosa, or systemic as noted in this example of indolent systemic mastocytosis. Clinical features of systemic mastocytosis include abdominal pain, diarrhea, pruritis and anaphylaxis. Darier sign describes the formation of a wheal when the skin is rubbed and dermatographism refers to an area of dermal edema that occurs in reponse to stroking the skin with a pointed instrument.

Systemic mastcytosis is a neoplasm in which the clonal population consists of mast cells with activating point mutations in the KIT tyrosine kinase receptor. It is characterized by compact aggregates of mast cells involving the bone marrow. The mast cells in systemic mastocytosis often display abnormal morphology such as spindling. The aggregates show paratrabecular distribution and are admixed with eosinophils and benign lymphocytes. Reticulin fibrosis is common and osteosclerosis may be seen. By immunohistochemistry, the mast cells often show aberrant expression of CD2 and/or CD25 in addition to expression of normal mast cell markers (tryptase and CD117).

This slide shows CD117 stain. See Related Content for H&E stain.